Conductive electrodes such as ruthenium oxide (RuO.sub.2) and lanthanum strontium cobalt oxide (La.sub.0.5 Sr.sub.0.5 CoO.sub.3 generally referred to as LSCO) have been extensively studied as electrode materials for thin film capacitors in which ferroelectric/paraelectric materials are used as dielectrics. Exemplary structures are shown in the following patents. U.S. Pat. No. 5,519,235 relates to a ferroelectric capacitor heterostructure wherein an amorphous SiO.sub.2 surface on a silicon wafer surface is initially primed or coated with a thin layer of titanium, tantalum or titanium dioxide and then coated with a thin layer of a metal such as platinum for the subsequent polycrystalline growth of metallic oxide electrode materials such as LSCO, RuO.sub.x, and SrRuO.sub.3. U.S. Pat. No. 5,270,298 relates to formation of crystalline metal oxide thin films such as LSCO employing a layered perovskite, such as bismuth titanate (BTO), template layer to initiate c-axis orientation in LSCO and PZT overlayers. Similarly, U.S. Pat. No. 5,248,564 used a layer of BTO on a silicon dioxide layer prior to a layer of LSCO. The interlayers of BTO are c-axis oriented only, i.e., uniaxially oriented.
Conductive RuO.sub.2, which has a rutile structure and tetragonal unit cell with a=b=0.44902 nanometers (nm), c=0.31059 nm, has been widely studied recently due to its unique properties compared to other oxide materials. High electrical conductivity, thermal stability, and chemical resistance make RuO.sub.2 very attractive in a variety of applications. Amorphous or polycrystalline RuO.sub.2 thin films have been deposited on a variety of substrates, such as oxidized silicon (SiO.sub.2 /Si), silicon (Si), quartz, glass, and magnesium oxide (MgO). Recently, epitaxial RuO.sub.2 thin films have been grown on lattice matched substrates such as LaAlO.sub.3, yttria-stabilized zirconia (YSZ), YSZ/Si, and sapphire. The growth of highly textured RuO.sub.2 on SiO.sub.2 /Si is more relevant in electronic devices since SiO.sub.2 is almost exclusively used as a field oxide, as a passivation layer, or as an isolation material in silicon-based circuitry. Highly textured RuO.sub.2 is preferable for use as electrodes in dielectric thin film capacitors because well oriented electrodes can further enhance the electrical and dielectric properties of dielectric materials. Nevertheless, all the previous RuO.sub.2 films deposited on SiO.sub.2 /Si show polycrystalline or uniaxial normal alignment (random in-plane orientation).
The conductive oxide La.sub.0.5 Sr.sub.0.5 CoO.sub.3 (LSCO), which has a psuedo-cubic lattice constant of 0.3835 nm and a room temperature resistivity of 90 .mu..OMEGA.-cm, has been extensively studied as an electrode material for ferroelectric thin film capacitors, where the dielectric materials can be PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT) or lanthanum-modified PZT. The improved device performance obtained by using LSCO as an electrode material, compared with the use of conventional platinum, has been attributed to the better structural/chemical compatibility and the cleaner interface (less charged defects) between LSCO and the dielectric materials. Fewer oxygen vacancies within the near interface region of the ferroelectric layer may also contribute to superior device performance.
For applications of LSCO films such as electrodes for nonvolatile ferroelectric random access memories (NFRAMs), epitaxial and/or well-textured LSCO films are preferable. The reduced grain-boundary scattering from an epitaxial LSCO film leads to lower resistivity of the film, which is a prerequisite for high frequency applications. As a bottom electrode and/or seed layer for ferroelectric thin film capacitors, well textured LSCO films also induce epitaxial or preferential oriented growth in subsequently deposited ferroelectric films. This is important since a highly oriented ferroelectric layer can produce a larger remnant polarization compared to a randomly oriented ferroelectric layer.
Epitaxial and/or well-textured LSCO films have been grown on SrTiO.sub.3, MgO, LaAlO.sub.3 and yttria-stabilized zirconia (YSZ). The growth of well-textured LSCO on technically important SiO.sub.2 /Si is more relevant in microelectronic devices since SiO.sub.2 is almost exclusively used as a field oxide, a passivation layer, and/or an isolation material in silicon-based circuitry. Highly oriented LSCO on SiO.sub.2 /Si has been accomplished by using Bi.sub.4 Ti.sub.3 O.sub.12 as a template (see U.S. Pat. No. 5,248,564). Nevertheless, the LSCO films deposited on SiO.sub.2 /Si by this method show only uniaxial normal alignment with random in-plane orientation. The growth of well-textured or biaxially oriented LSCO films (both normal to and in the film plane) on SiO.sub.2 /Si has not previously been accomplished.
Improved electric/dielectric properties of PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT), BaTiO.sub.3, SrTiO.sub.3, and B.sub.x Sr.sub.1-x TiO.sub.3 have been observed by using LSCO and/or RuO.sub.2 as the electrode materials, as compared to the use of conventional platinum electrodes. Both amorphous and/or polycrystalline thin films of RuO.sub.2 or LSCO have been previously deposited on SiO.sub.2 /Si substrates. The growth of highly oriented or well textured RuO.sub.2 and LSCO thin films on SiO.sub.2 /Si substrates is preferable for electrodes in dielectric thin film capacitors as such highly oriented or well textured electrodes can further enhance the electrical and dielectric properties of subsequently deposited dielectric materials.
Existing technology does not provide highly oriented conductive oxides on substrates such as amorphous SiO.sub.2 /Si and glass. The difficulties of forming such oriented layers on amorphous or polycrystalline substrates are due to seed growth. A structure of epitaxial RuO.sub.2 /YSZ/SiO.sub.2 /Si (with a SiO.sub.2 layer greater than 100 nm in thickness) has been previously achieved by additional high temperature processing steps (see U.S. Pat. No. 5,912,068 by Jia for "Epitaxial Oxides on Amorphous SiO.sub.2 on Single Crystal Silicon". Such high processing temperatures (greater than 900.degree. C.) present serious problems for processing on silicon and glass. For example, such a high temperature process cannot be used where there are active devices located on the silicon or where the melting temperature of the substrate is lower than the processing temperature.
It is an object of the present invention to provide a method of forming highly oriented conductive oxides on SiO.sub.2 /Si substrates, such highly oriented conductive oxides preferably characterized as biaxially oriented.
Another object of the present invention is to provide a low temperature method of forming highly oriented conductive oxides on SiO.sub.2 /Si substrates.
Another object of the invention is to provide a thin film structure including a thin layer of biaxially oriented YSZ on a SiO.sub.2 /Si substrate for subsequent deposition of highly oriented conductive oxides, said thin layer of oriented YSZ formed by ion-beam-assisted-deposition (IBAD).
Still another object of the present invention is to provide a thin film structure including a structure of an oriented layer of Ba.sub.0.5 Sr.sub.0.5 TiO.sub.3 (BSTO) and/or Ba.sub.1-x Sr.sub.x TiO.sub.3 0.ltoreq.x.ltoreq.1) on a biaxially oriented layer of RuO.sub.2 on an ion-beam-assisted-deposited (IBAD) layer of YSZ on a Si.sub.2 /Si substrate.
Yet another object of the present invention is to provide a thin film structure including a structure of a biaxially oriented layer of La.sub.0.5 Sr.sub.0.5 CoO.sub.3 on a biaxially oriented layer of CeO.sub.2 on an ion-beam-assisted-deposited layer of YSZ on a SiO.sub.2 /Si substrate.